The present invention relates to compositions for improved ceramic armor. Dense silicon carbide ceramics have been shown to be an effective means to protect against a wide variety of ballistic threats because of their combination of high hardness, strength and stiffness with low bulk density and favorable pulverization characteristics upon impact. Dense silicon carbide is typically produced using the following method steps:                1. Mixing fine silicon carbide powders with a sintering aid.        2. Drying the powder and if necessary adding processing aids to it.        3. Green forming the powders into a shape so that individual powder grains are in close contact with each other and the shape is retained after forming. Common forming techniques are slip casting, dry pressing, and extrusion.        4. Removing the processing aids such as binders by heat treatment.        5. Densification of the ceramic part by sintering, hot pressing, hot isostatic pressing (HIP) or sinter+HIP in a controlled atmosphere.        
Silicon carbide is a covalently bonded ceramic having low self-diffusion coefficients. To increase diffusion and facilitate densification, sintering aids are added in amounts generally less than 5 volume percent. For silicon carbide, the first additive used for densification was Al2O3 and the method for densification was hot pressing. This was developed in the 1950s as reported by the Journal of the American Ceramic Society, Volume 39, pp. 386-89 (1956) in the article by Alliegro et al. titled “Pressure-Sintered Silicon Carbide.” Effective use of Al2O3 was also reported by the Journal of Material Science, Vol. 10, pp. 314-320 in the article by F. F. Lange titled “Hot-Pressing Behavior of SiC Powders with Additions of Alumina,” and was shown by Lange to be due to liquid phase sintering. In 1970s and 1980s, pressureless sintering was developed by using compounds based on elements such as Boron (B), (S. Prochazka, “The Role of Boron and Carbon in Sintering of Silicon Carbide,” Special Ceramics, Vol. 6, edited by P. Popper, Stoke-On-Trent, England, 1975, pp. 171-181), Carbon (C), Beryllium (Be), (U.S. Pat. No. 4,172,109), and Aluminum (Al), (D. H. Stutz, S. Prochazka, J. Lorenz, “Sintering and Microstructure Formation of Beta-SiC,” J. Am. Ceram. Soc., 68[9], 479-82, (1985). These additives were added for the purpose of promoting solid-state diffusion. Carbon was added for the purpose of cleaning off the SiO2 layer from the silicon carbide surfaces and allowing the surfaces to be activated by B, Be and Al.
In the 1980s and 1990s, work was done on liquid phase sintering of SiC using rare earth oxide additives. This was disclosed in U.S. Pat. Nos. 4,564,490 and 4,569,921 and in an article by L. Cordrey, et al. titled “Sintering of Silicon Carbide with Rare-Earth Oxide Additions,” in Sintering of Advanced Ceramics, v. 7, (1990), pg. 618, edited by C. A. Handwerker, et al. For these materials, the diffusion occurs through the liquid phase instead of through the solid phase. For successful liquid phase sintering, Negita stated that free energy of formation for the metal oxide additives must be more negative than free energy of oxidation for silicon carbide at sintering temperatures. See K. Negita, “Effective Sintering Aids for Silicon Carbide Ceramics: Reactivities of Silicon Carbide with Various Additives,” J. Am. Ceram. Soc., 69[12], C-308C-310, (1986). By using oxide additives that meet this criterion, the oxide additives remain stable and do not result in oxidation and decomposition of the silicon carbide. Oxidation and decomposition of the silicon carbide, besides resulting in lost material, produces gas species that can inhibit sintering. Shown below are the reactions for silicon carbide decomposition along with the temperature in which they have the lowest free energy. It is seen that reaction products change with temperature.
1) SiC + O2 → SiO2 (s, l) + C (300   1800° C.)2) ⅔SiC + O2 → ⅔SiO2 (s, l) + ⅔CO(1800   2075° C.)3) SiC + O2 → SiO (g) + CO(2075   2100° C.)4) 2SiC + O2 → 2Si (s, l) + 2CO(2100   2600° C.)
The free energy versus temperature is shown in FIG. 1. FIG. 2 shows the free energy of formation for rare earth oxides in comparison to reaction c from 1800-2600° K. Rare earth oxides it is seen are stable versus silicon carbide. As such, much emphasis was placed on the sintering behavior of these materials in the 1990s.
In early 1990s, Andre Ezis at BAE Systems Advanced Ceramics showed that the choice of sintering additive was important in determining ballistic performance. See U.S. Pat. No. 5,354,536. The use of AlN as a sintering additive was shown to result in clean grain boundaries even in grades of SiC with metal impurities and the optimum amount of sintering additive was found to be a function of the surface area. The fracture of these materials was intergranular. The superior ballistic performance of these materials suggests the importance of fracture mechanism and microstructure in determining ballistic behavior. It should be noted that the ballistic event involves significant pulverization of the ceramic. The pulverization characteristics of ceramics, in which a significant mass of material is comminuted into fine particles underneath the projectile, have not been related to static mechanical properties. Static mechanical properties when applied to ballistic properties have generally been applied to cracks forming near the surface of the ceramic during impact.
In SiC for static applications, second phase additions have been added to improve toughness. Specifically, M. Janney, “Mechanical Properties and Oxidation Behavior of a Hot-Pressed SiC-15 vol %-TiB2 Composite,” Ceramic Bulletin, Vol. 66[2], 322-324, (1987), determined an increase in toughness from 3.1 to 4.3 MPa m ½ with 15 volume percent TiB2 additive (20 weight percent) and G. C. Wei and P. Becher, “Improvement in Mechanical Properties in SiC by Addition of TiC Particles,” Journal American Ceramic Society, 67[8] 571-74 (1984), determined an increase in fracture toughness for different volume percent TiC additive. These carbides and borides are stable versus SiC at high temperatures and have been shown to toughen the material by crack deflection.
In further work, V. D. Krstic and M. Vlajic, U.S. Pat. No. 5,470,806, patented a powder bed technology to liquid phase sinter SiC with transition metal oxide additives for the purpose of fusing the oxides and converting them into carbides during the course of sintering. The powder bed surrounding the part contained silicon carbide, aluminum oxide and carbon and facilitated the conversion to carbides in the sintered body and prevented excessive weight loss during sintering. The part and the powder bed were contained in a sealed graphite crucible. Aluminum oxide was used as the sintering additive to promote rapid densification and minimize reaction times. For an SiC composition containing 6.5 wt. % Al2O3, 2.5 wt. % TiO2, 6.0 wt. % ZrO2 and 2.0 wt. % C, a fracture toughness of 6.3 Mpa m ½ could be achieved while for an SiC composition containing 8.7 wt. % Al2O3, 20.0 wt. % TiO2, 6.3 wt. % ZrO2 and 5.0 wt. % C, a fracture toughness of 7.2 MPa m ½ could be achieved. In these materials, the TiO2 and ZrO2 reacted to TiC and ZrC during sintering. Typical sintered SiC has a fracture toughness of 4.0 to 5.0 MPa m ½.
In an effort to increase the ballistic performance of SiC, Applicants have looked at oxide additives that do not meet Negita's criterion for use as sintering aid and result in carbide formation below or at sintering temperatures. Oxide additives that do not meet Negita's criterion cause the decomposition of SiC by either simultaneous formation of metal carbide, silicon and CO or simultaneous formation of silicon, metal and CO. The generalized equations are shown below.2SiC(s)+aMvOw(s,l)—2Si(s,l)+bM(s,l)+2CO(g) 2SiC(s)+cMvOw(s,l)—dSiO(g)+eMXCy(s,l) 
In the present invention, ZrO2 additions are added which will result in decomposition of the SiC and formation of ZrC, and some combination of Si, SiO and CO. The present invention demonstrates that ZrO2 can be used to increase ballistic performance of silicon carbide when added in small amounts and using a furnace design in which the atmosphere can be controlled. Both ZrO2 and stabilized ZrO2 such as 3 mole percent Y2O3 stabilized ZrO2 (3TZY) can be used.
Along with decomposition of SiC and ZrO2 during sintering of SiC, volatization reactions involving the SiO2 (native silica on the SiC grain) and ZrO2 to SiO, ZrO, and O2 would be expected in SiC ceramics. The vapor pressure of these species depends on temperature, the amount of SiO2, ZrO2, free carbon, other phases and kinetic considerations. The presence of these species however would not affect the direction of the high temperature decomposition reactions at sintering temperatures of SiC in inert gas or vacuum furnaces.